Ultrastructural studies have revealed increased permeation of the blood-brain barrier by protein in Chinese hamsters after exposure to 2450-MHz microwaves (CW) at 10 mW cm−2. Reversibility of this microwave-induced permeability in microvasculature of Wistar rats and Chinese hamsters was assessed. The animals were anesthetized and their heads were irradiated in the near field with 2800-MHz CW microwaves at 10 mW cm−2 for two hours. Horseradish-peroxidase protein was injected as a tracer to study its transport across capillaries in all regions of the brain. Gross- and electron-microscopic observations indicated that the relative impermeability of the blood-brain barrier is re-established within two hours after irradiation. A significant but incomplete increase of permeability was observed one hour after exposure. Preliminary data indicate that the blood-ocular barrier was similarly affected.